The present invention relates to a microcomputer having a CPU (central processing unit) and a nonvolatile memory capable of being electrically written and erased, and relates in particular to a technology effective on one-chip microcomputers having flash memories for permitting or prohibiting programming and erasure on nonvolatile memories.
Electrically erasable and programmable nonvolatile memories such as flash memories, store information according to differences in threshold voltages programmed into the memory cells. This differential in threshold voltages in the flash memory is implemented by differences in the amount of electrons and positive holes stored by the floating gates. For example, applying a voltage at a high level relative to a threshold voltage in a stable thermal state is called the program status; and applying a voltage at a low level relative to the threshold voltage is called the erase status (The reverse of this definition may also be true.) There are no particular restrictions at this time but the erase operation to set the memory cell to erase status and the programming operation to set the memory cell to program status involve repeatedly applying a high voltage pulse and verifying the resulting threshold voltage. The writer mode and the boot mode are operating modes to allow programming and erasing the on-chip flash memory of the microcomputer. The writer mode is an operating mode treating the microcomputer as equivalent to a flash memory chip to program and erase the memory by connecting to a programming device such as an EEPROM writer. The boot mode is an operating mode for example, to establish communications to allow programming or erase with the microcomputer installed the system, by way of synchronized start-stop or a nonsynchronized serial interface (UART). The writer mode can be utilized to program data or program in an initialized state on the on-chip memory, prior to installation in the system. The boot mode on the other hand, can be utilized to reprogram information stored in the on-chip flash memory for program version upgrades or tuning data, prior to installation in the system.
The boot mode of the related art utilizes a serial interface as the basic interfacing method so a serial interface circuit such as for start-stop synchronizing, was incorporated into the system board of the microcomputer, when doing on-board programming by using the boot mode.